In general, a centrifugal separator, includes a rotor, which is rotatable around a rotary axis, and a casing, which forms an outer space for the rotatable rotor. The rotor includes a rotor wall, which defines an outer periphery of the rotor and encloses, an inner space, at least an inlet, which extends through the rotor wall and permits feeding of a product to the inner space, and a number of outlets, which extend through the rotor wall and are arranged to permit intermittent discharge of a separated product from the inner space to the outer space. The outlets are distributed around the periphery of the rotor.
During operation of such a centrifugal separator, sounds are generated, and more precisely, the rotor may whistle, i.e. sound generated in cavities of the rotor. The sound level of such whistle may sometimes be unacceptably high and exceed the level of structural sounds from other sound sources of the centrifugal separator, for instance from turbulent pressure fluctuations in said outer space and internally in the rotor, and from the transmission and the motor.
Whistle arises in the cavities in the rotor wall, which are formed by said outlets, due to the fact that the cavities, which enclose a gas volume, have a velocity in relation to the gas, which is located outside the cavities in said outer space. Whistle is excited by this gas, normally air, when it passes the orifices of the cavities towards the outer space and causes the gas located in the cavity to vibrate. The whistle tendency increases generally with an increasing rotary speed of the rotor.
There is a relation between the presence of whistle and the geometry of the cavities, and more precisely the frequency of such whistle is determined by the shape and dimensions of the cavities. Whistle may thus be limited by a change of this geometry. The possibilities of making such geometrical changes are limited by the strength requirements, which are set up for the material portions, which are present between adjacent cavities and which connect an upper part of the rotor to a lower part thereof.
SE-B-463 903 discloses such a centrifugal separator. The centrifuge rotor includes a number of outlets, which extend through the rotor wall for discharging a separated product from the inner space of the rotor to an outer space outside the rotor. The outlets are evenly distributed around the periphery of the rotor. In order to disturb the formation of sound waves in the outlets, projections in the form of elongated ribs are provided between the outlets.
SE-C-505 385 discloses a similar centrifugal separator with a rotor having a number of outlets, which are evenly distributed around the periphery of the rotor. In this document, a geometrical change of said outlets are proposed in order to provide a reduction of the generated sound level. More precisely, outlets are disclosed, which have a decreasing cross-section area from the outer side of the rotor wall to a position between the outer side and the inner side.